How to Add a User to a Group in Linux

When you need to give a Linux user access to Docker, sudo, shared directories, or other protected resources, you usually add the user to a group. Groups let you manage permissions for several users at once instead of changing permissions for each account separately.

This guide explains how to add an existing user to a Linux group, add a user to multiple groups, change a user’s primary group, and verify that the change was applied.

There are two types of groups:

• Primary group: assigned to a user when the account is created. Usually, the name of the primary group is the same as the username. When a user creates a file, the file’s group is set to the user’s primary group. The primary group is stored in the /etc/passwd file.
• Secondary (supplementary) groups: used to grant additional permissions. For example, adding a user to the docker group allows them to run Docker commands without sudo.

Each user has exactly one primary group and can belong to zero or more secondary groups.

Only root or users with sudo privileges can modify group membership.

Quick Reference #

Displaying User Groups #

Before modifying groups, it is useful to check a user’s current group membership.

The id command displays the user’s UID, primary group, and all secondary groups:

id linuxize

uid=1000(linuxize) gid=100(users) groups=100(users),10(wheel),95(storage),98(power),990(libvirt),993(docker),999(kvm)

The groups command prints only the group names:

groups linuxize

wheel storage power users libvirt docker kvm

If you omit the username, both commands display information for the currently logged-in user.

Listing All Groups on the System #

To list all groups on the system:

getent group

To list all members of a specific group:

getent group docker

docker:x:993:linuxize,deploy

Adding a User to a Group #

Before changing group membership, make sure both the user and group already exist. The usermod command does not create missing users or groups for you.

Check the user with getent passwd:

getent passwd linuxize

Check the group with getent group:

getent group docker

If both commands return entries, you can add the user to the group.

To add an existing user to a secondary group, use the usermod -a -G command followed by the group name and the username:

sudo usermod -a -G groupname username

For example, to add the user linuxize to the docker group:

sudo usermod -a -G docker linuxize

On success, the command produces no output. Verify the new membership with id:

id linuxize

uid=1000(linuxize) gid=1000(linuxize) groups=1000(linuxize),993(docker)

Group changes apply after the user logs out and logs back in. To apply the change in the current session, run:

newgrp docker

The newgrp command starts a shell with the selected group as the active group. For normal login sessions, logging out and signing back in is the cleaner option.

Adding a User to Multiple Groups #

To add a user to multiple secondary groups at once, separate the group names with commas (no spaces):

sudo usermod -a -G group1,group2,group3 username

For example, to add linuxize to the docker and developers groups:

sudo usermod -a -G docker,developers linuxize

Do not put spaces after the commas. usermod expects one comma-separated list.

Changing a User’s Primary Group #

To change a user’s primary group, use usermod with the lowercase -g option:

sudo usermod -g groupname username

For example, to change the primary group of the user linuxize to developers:

sudo usermod -g developers linuxize

Note the difference: -G (uppercase) sets secondary groups, -g (lowercase) sets the primary group.

Creating a User with Groups #

The useradd command can assign both primary and secondary groups when creating a new user:

sudo useradd -g users -G wheel,developers nathan

This creates a user nathan with users as the primary group and wheel and developers as secondary groups.

Removing a User from a Group #

To remove a user from a group, use the gpasswd command with the -d option:

sudo gpasswd -d username groupname

For example, to remove linuxize from the docker group:

sudo gpasswd -d linuxize docker

Creating and Deleting Groups #

To create a new group :

sudo groupadd groupname

To delete a group :

sudo groupdel groupname

You cannot delete a group that is a user’s primary group. Change the user’s primary group first.

Troubleshooting #

Group changes do not apply in the current shell
Linux reads a user’s groups when the session starts. If id username shows the new group but the user still cannot access the resource, log out and log back in. For a temporary shell with the new group, run newgrp groupname.

usermod: user 'username' does not exist
The username is missing or spelled differently. Check the account with getent passwd username. If you need to create the user first, use useradd or your distribution’s user management tool.

usermod: group 'groupname' does not exist
The group must exist before you can add a user to it. Check it with getent group groupname. If it is missing, create it with sudo groupadd groupname, then run usermod -a -G groupname username again.

The user lost access to other groups after running usermod -G
This happens when -G is used without -a. The command replaces the user’s secondary groups with the groups listed after -G. Add the missing groups back with sudo usermod -a -G group1,group2 username, then verify with id username.

FAQ #

What happens if I forget the -a flag with usermod -G?
The user will be removed from all secondary groups except those listed in the command. This is the most common mistake when managing groups. Always use -a -G together.

Do group changes take effect immediately?
No. The user must log out and log back in for the new group membership to take effect. You can verify by running id username; the change shows up there immediately, but the user’s active session still uses the old groups.

What is the difference between -g and -G in usermod?
Lowercase -g sets the user’s primary group. Uppercase -G sets secondary (supplementary) groups. When used with -a, -G appends to the existing list instead of replacing it.

How do I find out which groups exist on the system?
Run getent group to list all groups, or getent group groupname to check whether a specific group exists and see its members.

Can a user belong to multiple primary groups?
No. Each user has exactly one primary group. To grant access to multiple resources, use secondary groups.

Conclusion #

The usermod -a -G command adds existing users to secondary groups in Linux. Use id, groups, and getent to verify the change, and always include -a when working with -G so existing group memberships stay intact.

About the authors

Dejan Panovski

Dejan Panovski is the founder of Linuxize, an RHCSA-certified Linux system administrator and DevOps engineer based in Skopje, Macedonia. Author of 800+ Linux tutorials with 20+ years of experience turning complex Linux tasks into clear, reliable guides.

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